1. Technical Field.
The present invention relates generally to a semiconductor laser package and more particularly to a semiconductor laser package having an integral impedance matching transformer.
2. Discussion of Related Art
Semiconductor lasers are commonly used as optical sources in long distance communication systems for producing analog or digital modulated optical signals for transmission over fiber optics link. Typically, the laser is driven by a laser drive source which provides a DC current for biasing the laser and a high frequency modulation current which modulates the laser optical output at frequencies of 50 MHz to 1 GHz or higher. At these operating frequencies, any electrical connections of significant length must be considered as transmission lines and the circuit design must include considerations such as stray capacitive effects and the compatibility of the impedance of the driving and receiving components.
Typically, the RF drive circuit which produces the modulation signals has an output impedance which is several times higher than the impedance of the semiconductor laser. To avoid transmission line reflection problems due to the impedance mismatch of the laser and the RF drive source, several techniques have been employed to interface the laser to the laser drive source with impedance matching circuitry.
A well known and traditional approach is to place a resistor Ri having a resistive value equivalent to the difference between the drive circuit and the laser impedance, as shown in FIG. 1. Also shown in FIG. 1 is a typical laser drive source having a DC current source Idc being fed through a coil Ldc which is combined in a bias tee format with the RF current drive hn through resistor Rm and capacitor Cc. At operating frequencies of 50 MHz to 1 GHz, the RF drive circuit typically has an output impedance of around 50 to 75 ohms. As an example, if the impedance of the RF drive circuit is 75 ohms and the laser is 5 ohms, an Ri value of 70 ohms would impedance match the RF drive circuit to the laser.
A drawback of such an approach is the loss of RF energy dissipated by the resistor Ri, which must be compensated by higher RF drive requirements at the laser drive source. With the resistive matching technique such as in FIG. 1, the placement of the resistor Ri within the laser package or outside the laser package has virtual insignificant effects on the high frequency signal characteristics of the drive signals present at the laser.
U.S. Pat. No. 5,216,393 to Wandel proposes an impedance matching approach by use of a transformer. The transformer includes multiple windings which match the output impedance of the RF drive circuit and the input impedance of the semiconductor laser. Compared to the resistive impedance matching approach, the transformer dissipation of RF energy is much less, typically 4 dB less than the resistor.
The transformer impedance matching approach works well except at higher operating frequencies, such as at around 500 MHz to 1 GHz, the output of the laser tends to exhibit a negative sloping characteristic due to the transformer. The sloping is typically around 2 dB degradation in current amplitude. Another important consideration is the placement of the transformer. If the transformer is not disposed close to the semiconductor laser, as would be the case if the transformer is external to a semiconductor laser package, the parasitic effects from the stray inductance and capacitance at higher operating frequencies will become more pronounced and problematic.
Therefore, there exists a need for a semiconductor laser package having impedance matching circuitry which matches the impedance of the RF drive circuitry and the impedance of the semiconductor laser while minimizing the resistive loss and the parasitic capacitive and inductive effects.